The proposed research is designed to elucidate mechanisms of the immune response of both susceptible and refractory mosquitoes to filarial worms. Specific information will be gained on mechanisms of immune recognition, activation, and inhibition in Aedes aegypti (black-eyed Liverpool strain), Ae. aegypti (Rockefeller strain) and Ae. trivittatus against Brugia pahangi, B. malayi and Dirofilaria immitis. Experiments are designed to determine whether differences in phenoloxidase activity and (or) activation of prophenoloxidase in hemolymph and (or) hemocytes from mosquitoes correlate with known differences in immunological competence, and to determine whether such differences reflect changes in the concentration of melanin precursors. Lectin binding specificities and lysosomal enzyme markers will be used to assess subpopulations of hemocytes and to identify changes associated with hemocyte activation. Transmission electron microscopy will be used to characterize melanization processes in Ae. aegypti as compared to Ae. trivittatus. Studies also are designed to develop a method to quantitate melanin deposits on parasites by using radiolabeled tyrosin inoculations into host mosquitoes. The overall aim of this project is to elucidate mechanisms of mosquito immunity, thereby increasing our understanding of the epidemiology of mosquito-borne filariasis.